The inexpensive and continued production of hydrocarbons is essential to the maintenance of modem society. In view of a limited world hydrocarbon supply, keeping energy costs low requires a continual improvement in geological formation evaluation and hydrocarbon recovery. This constant quest for improvement requires breakthroughs in well logging technology. Improved formation data logs allow more accurate predictions of where producible hydrocarbon may be found and increase the yield at these sites. Past improvements in the field of hydrocarbon well logging include induction and resistivity tools, acoustic tools, and nuclear tools.
The determination of whether a particular geological formation contains produceable hydrocarbon can be extraordinarily complicated. Initially, it must be determined what, if anything, a sub-surface formation contains. If it contains fluid, it must be determined whether this fluid is water, hydrocarbon, or both. One difficulty encountered by the hydrocarbon industry is its need to retrieve a hydrocarbon stream from the ground that contains only a limited supply of water or brine. Thus, although an area may contain adequate hydrocarbons, excessive water may make it unsuitable for production. Resistivity tools have been useful in determining whether water is present in an hydrocarbon-rich formation. However, the mere presence of sub-surface water does not give a full picture of whether there exists producible hydrocarbon. This also depends upon the character of the detected water. Thus, resistivity tools are not ideal because they indicate merely the presence of water, and cannot indicate its mobility. When underground water comes up-hole with the retrievable hydrocarbons it is known as being free, movable, or reducible. Conversely, when the underground water remains down-hole at the time of production it is known as being bound, immovable, or irreducible. Thus, if one cannot determine the mobility of the underground water, many potentially productive hydrocarbon zones with high irreducible water saturation are bypassed because of fear of excessive water production.
One technology that has proved to be helpful in modem formation evaluation is nuclear magnetic resonance (NMR) technology. This technology assists in the control of water production and identification of pay zones with high irreducible (or bound) water saturation. One such NMR tool is the MRIL.RTM. C-type tool, shown in FIG. 1. Also shown is a borehole 150. The MRIL.RTM. apparatus is a centralized device containing a permanent magnet and a radio frequency (RF) pulse generator (not shown). The tool as shown has an outer diameter 110 of 6" and a length of about 50'. A slim version of the tool (not shown) has an outer diameter of 4 1/2". In an 8" borehole 150 , MRIL.RTM. depth of investigation 120 is 4 inches. The tool's permanent magnet generates a magnetic field of 2500 gauss (5,000 times the strength of the earth's magnetic field) with a field gradient of 17 gauss/centimeter. When random hydrogen nuclei interact with the applied magnetic fields, measurable signals are produced. The primary field of the permanent magnet aligns the hydrogen nuclei in one direction. The tool then uses its radio frequency generator to pulse a second magnetic field perpendicular to the permanent magnet's primary field. This RF generator operates at the Larmor frequency to rotate the nuclei 90.degree. with respect to the alignment induced by the permanent magnet. After the RF pulse is turned off, the nuclei gradually dephase or disorder, causing the signal to decay. MRIL.RTM. operates on three close frequencies, which improves the signal to noise ratio and increases the logging speed. The time consumed by the nuclei to completely dephase is called the T2 time, and the time required for the nuclei to return to their initial aligned position is called the T1 time. The T2 time is shorter than the T1 time and has been chosen as the time measured by the current MRILX C-type tool.
This T2 time varies from one hydrogen nucleus to another, depending on the location of the hydrogen in the formation. When the hydrogen is located adjacent an underground rock surface, it comprises immovable or bound water. Surface tension holds this water to the rock surface and causes the water to remain downhole. When this bound fluid is affected by the magnetic field of an NMR tool, the rock causes the bound water to have a shorter T2 time. Moveable water, in contrast, lives in the bulk, and not at the surface of a rock. Thus, the T2 time of its hydrogen is unaffected by a rock's surface and so is longer in duration. In this way, movable water may be differentiated from immovable water based on their respective T2 times.
FIG. 2 is a graph illustrating T2 data. T2 data has two important aspects, known as the T2 distribution 200 and the T2 cut-off 210. The T2 cut-off 210 separates the effective porosity into irreducible porosity 220 and moveable porosity 230. In other words, the T2 cut-off is the dividing line between the bound and the free sub-surface water. In contrast, the T2 distribution is used to calculate a distribution of porosity components as a function of their T2 times. Thus, in the T2 distribution, the sum of all porosities whose T2 time is less than the T2 cut-off yields the NMR-bulk volume of irreducible water (MBVI). Similarly, the sum of all porosities whose T2 time is greater than the T2 cut-off furnishes the NMR-determined free fluid index (MFFI). The NMR determined effective formation porocity (MPHI) is then found by adding MBVI and MFFI. The T2 cut-off may not, however, be derived from the T2 distribution. Thus, while the n distribution at a particular depth may be derived by the readings of an NMR tool, the determination of the T2 cutoff for a core sample at this time requires laboratory analysis. Besides porosities, NMR measurements also provide better estimates of formation permeabilities than can be derived from conventional logs.
Nonetheless, prior art NMR measurement techniques suffer from significant shortcomings. For instance, the T2 cut-off time may vary significantly along the length of a well bore. The prior art ignored such variation and arrived at a single T2 cut-off point by averaging the T2 cut-off times from a number of core samples taken from the borewall. Those in the industry would prefer a more accurate method for determining the T2 cut-off times. A more accurate method would allow refinement of geological formation evaluation so that areas containing significant irreducible water could be produced whereas nearby areas containing significant movable water could be avoided. Ideally, such a method would cost a minimal amount. For example, the method would minimize the number of core samples that should be taken and consolidate into one period the time required for expensive production.